The invention relates to a method for manufacturing a light wave guide comprising a cladding and a light-conducting core, from a glass tube which on or below its interior surface comprises a material forming the core of the finished light wave guide, the material containing at least one substance which is selectively diffused out at a suitable temperature from the material which later forms the core into the interior space of the glass tube.
One of the most important processes for the manufacture of light guiding fibers is the interior coating of a tube (MCVD, PCVD, PICVD). Thereby thin layers of doped quartz glass are precipitated on the inner side of a quartz glass tube. The desired radial course of the refractive index is thereby adjusted by the choice and the concentration of the doping agent, the core thereby showing a higher refractive index than the cladding.
After the coating the tube is collapsed in several steps at an elevated temperature to form a rod, which can be drawn out to form a light-conducting fiber. It is yet possible to draw a light-conducting fiber directly from the coated tube without collapsing the tube to a rod.
From DE-OS No. 29 12 960 a method for the manufacture of light-conducting fibers is known whereby a tube is used, being formed of glass at least in its inner part, comprising at least one component to decrease the refractive index, whereby the tube is heated in such a way that a part of this component diffuses out through the inner side of the tube while the tube is drawn out to form a light-conducting fiber.
The diffusing-out of the component decreasing the refractive index and the heating up to the drawing-temperature thereby substantially takes place in a single step; the drawing-out of the tube to form a light-conducting fiber can take place in a second step as well.
This method shows the disadvantage that a constant course of the refractive index profile along the whole length of the so manufactured fiber cannot be obtained. Since the tube is sealed at one end at the beginning of the diffusing-out process, a counter-pressure arises in the interior space of the glass tube by the diffusing-out doping agent, so rendering difficult a further diffusing-out process. If this pressure reaches the equilibrium pressure no further diffusing-out takes place. An influence on the refractive index profile is expensive and only possible on the heat input and the drawing speed, respectively. This also applies if, in a first step, solely the diffusing-out and, in a second step, the drawing-out of the tube takes place.
This method therefore cannot be applied to a manufacture of light-conducting fibers having optical characteristics being independent from the length.